The present invention relates generally to the fields of inductively coupled plasma (ICP) cryogenic etching of silicon to produce textured surfaces, and battery anodes employing such textured surfaces. Specifically, it concerns a lithium ion battery anode employing textured silicon, a method to produce such textured silicon, and lithium ion batteries utilizing such textured silicon anodes.
The energy demands of emerging portable electronic devices, plug-in hybrid electric vehicles and even space applications remain unfulfilled. More advanced energy storage devices are needed to accommodate the additional features in our portable devices, to minimize the emissions of greenhouse gases from automobiles or to enable more challenging space missions. The conventional lithium-ion cells utilize carbonaceous materials (nano-phase carbon, synthetic or natural graphite) as anodes, which have a maximum capacity of only 350 mAh/g, about one tenth of what would be available from the use of metallic lithium anode. The use of metallic lithium, however, is problematic mainly due to the safety and reliability issues associated with its morphological changes upon cycling. The focus has therefore been on the use of lithium alloys, as a compromise. Among the various elements that can be used as (alloy) anodes for lithium, silicon is the most promising system, because of its ability to react with 4 Li atoms per Si atom, which results in an impressive theoretical capacity of 4200 mAh/g.
The use of silicon for the anode of Li ion batteries is attractive, as it has the highest theoretical charge capacity of any material when used as an anode in a Li ion cell. However, the large volume expansion/contraction of bulk Si upon absorption and desorption of Li ions results in pulverization of the anode after several charge and discharge cycles.
U.S. Pat. Nos. 7,02,829 and 7,683,359 to Green (Green) are generally directed to a silicon/lithium battery to be produced as an integrated unit on a chip. The battery includes an anode formed from an array of submicron structures including silicon fabricated on a substrate and a cathode including lithium. Green is similar to numerous references in which the textured surface, in this case an anode, comprises “nano-wires” which are grown, etched, or otherwise attached to the surface. This approach, however fails to achieve the full potential of lithium ion batteries. The nano-wires are known to separate from the substrate and are random in arrangement on the surface.
U.S. Pat. No. 7,622,377 to Lee et al. (Lee) is generally directed to a microfeature workpiece substrate having through-substrate vias, and associated methods of formation. A method in accordance with one embodiment for forming a support substrate for carrying microfeature dies includes exposing a support substrate to an electrolyte, with the support substrate having a first side with a first conductive layer, a second side opposite the first side with a second conductive layer, and a conductive path extending through the support substrate from the first conductive layer to the second conductive layer. The method can further include forming a bond pad at a bond site of the first conductive layer by disposing at least one conductive bond pad material at the bond site, wherein disposing of at least one conductive bond pad material can include passing an electrical current between the first and second conductive layers via the conductive path, while the substrate is exposed to the electrolyte.
Lee is an example of a multitude of references which disclose various methods and approaches of etching silicon and other substrates to produce a desired effect. Lee and other similar references utilize a mask to produce the desired effect.
U.S. Pat. No. 6,033,928 to Eriguchi, et al. (Eriguchi) is generally directed to forming a silicon dioxide film on a silicon substrate and than forming hemispherical grains made of silicon, each having an extremely small diameter, which are deposited thereon by LPCVD. After annealing the hemispherical grains, the silicon dioxide film is etched using the hemispherical grains as a first dotted mask, thereby forming a second dotted mask composed of the silicon dioxide film. The resulting second dotted mask is used to etch the silicon substrate to a specified depth from the surface thereof, thereby forming an aggregate of semiconductor micro-needles. Eriguchi forms the substrate utilizing a “mask” in the form of the “dotted mask” applied to the silicon substrate.
U.S. Pat. No. 7,700,235 to Konishiike et al. (Konishiike) is generally directed to a battery capable of improving cycle characteristics. An anode active material layer is formed by a vapor phase method, and includes Si as an element. In the anode active material layer, a plurality of primary particles formed by growth in a thickness direction are included, and the plurality of primary particles are agglomerated to form a plurality of secondary particles. Each secondary particle is separated by a groove formed by charge and discharge, and some of primary particles are split particles split by the groove. The average number of the split particles per secondary particle in 5 or more adjacent secondary particles is 10 or more. Moreover, the primary particles and the secondary particles are inclined to the same side.
Konishiike utilizes an anode comprising a plurality of agglomerated materials on a substrate which dissipate the stress due to expansion and shrinkage according to charge and discharge. However, the anodes produced according to Konishiike do not achieve the full potential of lithium ion batteries.
U.S. Pat. No. 7,442,629 to Mazur et al. (Mazur) is generally directed to semiconductor substrates haying submicron-sized surface features generated by irradiating the surface with ultra short laser pulses. In one aspect, a method of processing a semiconductor substrate is disclosed that includes placing at least a portion of a surface of the substrate in contact with a fluid, and exposing that surface portion to one or more femtosecond pulses so as to modify the topography of that portion. The modification can include, e.g., generating a plurality of submicron-sized spikes in an upper layer of the surface. Mazur forms the various substrates by vaporizing parts of the surface. As such, the surface produced according to the disclosure is not crystalline, but is instead a plurality of amorphous domains. The substrate produced by Mazur have surface features with heights of less than about 1 micrometer.
U.S. Pat. No. 5,501,893 to Laermer et al. (Laermer) is generally directed to a method of anisotropic plasma etching of silicon to provide laterally defined recess structures therein through an etching mask employing a plasma, the method including anisotropic plasma etching in an etching step a surface of the silicon by contact with a reactive etching gas to removed material from the surface of the silicon and provide exposed surfaces; polymerizing in a polymerizing step at least one polymer former contained in the plasma onto the surface of the silicon during which the surfaces that were exposed in a preceding etching step are covered by a polymer layer thereby forming a temporary etching stop; and alternatively repeating the etching step and the polymerizing step. The method provides a high mask selectivity simultaneous with a very high anisotropy of the etched structures.
Other references include Marcel W. Pruessner, Williams S. Rabinovich, Todd H. Stievater, Doewon Park, and Jeffrey W. Baldwin “Cryogenic Etch Process Development for Profile Control of High Aspect-Ratio Submicron Silicon Trenches”, J. Vac Sci. Technol. B 25(1) (2007) pp. 21-28; referred to as Pruessner et al. (Pruessner.) Pruessner is generally directed to developments in the cryogenic dry-etch process to produce silicon photonics and MEMS/NEMS. A cryogenic etch process using low temperature (T less than or equal to −100° C. and SF6 and O2 gases is presented for fabricating high aspect ratio silicon microstructures, including photonic devices and micro- and nanoelectromechanical systems. The process requires only a single electron beam resist mask. Pruessner discloses optimized processes using low temperature (T=−110° and low chamber pressure (P=7 mTorr) which enable sidewall verticality greater than 89.5° with roughness of 1-10 nm. Pruessner discloses an alternative dry etch process to the so-called Bosch process. In the Bosch process, an etch step (SF6 gas) and a passivation step (C4F8) alternate every few seconds. The two steps are repeated to achieve the desired etch depth. In the process disclosed by Pruessner, the two processes occur simultaneously, at low temperatures (i.e., less than or equal to about −100° C.
Kanwar S. Nalwa, and Sumit Chaudhary, “Design of Light-Trapping Microscale-Textured Surfaces for Efficient Organic Solar Cells”, Optics Express, Vol. 18, No. 5, (2010) pp. 5168-5178; (Kanwar) is generally directed to light-trapping microscale-textured surfaces which absorb photonic energy. Kanwar disclosed substrates comprising a number of different layers which have been etched to produce a textured surface. The micro-textured surfaces have a height of about 2 microns and a pitch of about 1.5 microns.
Meint J. de Boer, J. G. E. Gardeniers, Henri V. Jansen, Edwin Smulders, Melis-Jan Gilde, Gerard Roelofs, Jay N. Sasserath, and Miko Elwenspock, “Guidelines for Etching Silicon MEMS Structures Using Fluorine High-Density Plasmas at Cryogenic Temperatures”, J. Micromech. Sys. Vol. 11, No. 4 (2002) pp. 385-401; (de Boer) is generally directed to the presentation of guidelines for the deep reactive ion etching (DRIE) of silicon MEMS structures, employing SF6/O2-based high-density plasmas at cryogenic temperatures. Procedures of how to tune the equipment for optimal results with respect to etch rate and profile control are described. Profile control is a delicate balance between the respective etching and deposition rates of a SiOF passivation layer on the sidewalk and bottom of an etched structure in relation to the silicon removal rate from unpassivated areas. Any parameter that affects the relative rates of these processes has an effect on profile control. The deposition of the SiOF layer is mainly determined by the oxygen content in the SF6 gas flow and the electrode temperature. Removal of the SiOF layer is mainly determined by the kinetic energy (self-bias) of ions in the SF6/O2 plasma. Diagrams for profile control are given as a function of parameter settings, employing the previously published “black silicon method”. Parameter settings for high rate silicon bulk etching, and the etching of micro needles and micro moulds are discussed, which demonstrate the usefulness of the diagrams for optimal design of etched features. De Boer thus presents general guidelines directed to avoiding formation of black silicon.
KR 20060117109 to Sun et al. (Sun) is generally directed to a silicon anode active material for a lithium secondary battery, which shows high electroconductivity and low electric resistance, undergoes a reduced change in volume upon charge/discharge, and imparts high output, high capacity and improved lifespan to a lithium secondary battery. The silicon anode active material for a lithium secondary battery is obtained by the method comprising the steps of: mechanically mixing and pulverizing silicon particles and cellulose-like vegetable fibers to allow the surface of the silicon particles to be coated with the vegetable fibers; and heat treating the silicon particles coated with the vegetable fibers under a reductive atmosphere or inert atmosphere to perform carbonization of the surface of the silicon particles.; The cellulose-like vegetable fibers are mixed with the silicon particles in a ratio of 0.1-10 per weight of the silicon particles.
Accordingly, there is a long felt need in the art to produce silicon anodes for lithium ion batteries which are not plagued by the effect of pulverization and other forces which limit the specific energy capabilities of the anode. Even if a fraction of this high capacity is realized and retained upon cycling, this will result in a significant enhancement in the specific energy. With a capacity of ˜1000 mAh/g, for example, the projected specific energy would allow for lithium ion batteries capable of over 200 Wh/kg.